Coordinate measuring machines (CMM) are used to determine the three-dimensional topography of an object. CMM typically comprise an arm movable in three directions (X, Y, Z) relative to a table supporting the object. Movement of the arm in any of these directions and thus the actual position of the arm with respect to the object is measured with suitable transducers.
For measuring the surface variations, measurement principles based on use of tactile sensors and of optical sensors are known.
In international patent application No. WO 89/07745, a probe head for use in coordinate measuring machines is disclosed. The probe head comprises a stylus which is supported for axial and angular displacements. A transducer senses axial forces on the sensing end of the stylus due to engagement of the stylus end with a workpiece. A strain gauge system provided on the stylus senses transverse forces on the sensing end of the stylus. The axial and transversal forces are used to determine the orientation of the surface of the workpiece, and a control system is described which responds to those forces to maintain the stylus normal to the workpiece surface during a scanning operation.
From the signals provided by the measuring transducers and from knowledge of the dimensions of the parts of the surface sensing device, a prediction can be made about the position of the centre of the stylus tip.
However, the stylus assembly is subject to bending due to contact with the workpiece surface and due to inertial forces while accelerating, and this bending makes the actual position of the centre of the stylus tip uncertain.
In U.S. Pat. No. 5,118,956, a scanning probe tip is disclosed, which is provided with a sensor, such as a mirror, an optical fiber or a bi-refringent element which changes state by vibrating or undergoing strain when a stylus connected to the probe contacts a workpiece. The sensor is provided on the stylus (in the case of the mirror) or in the stylus (in the case of the optical fiber). The change of state of the sensor due to surface contact with a workpiece causes a change in the path length, polarization state, or intensity of light waves conveyed by the sensor. As an example, an interferometer for detection of such changes is disclosed. Probe beams directed to the object to be investigated and returned beams may be propagated in optical fibers from the light source to the object and be returned in optical fibers to a detector. Optical sensors are not used to detect deformations or vibrations of the stylus by itself. Moreover the optical sensors replace classical electrical transducers.
As a disadvantage, this sensing configuration requires the recording of the variation of an interference pattern over a significant amount of time and/or the use of large-area, high-resolution position-sensitive detectors. Additionally a calibration of a change in interference patterns to the extent of stylus displacement and/or bending is difficult, as typically interference patterns are hardly exactly reproducible.
Scanning measurements based on tactile sensors, i.e. a workpiece-contacting tip, are generally associated with a principle problem. If the stylus is designed very inflexible/stiff, the sensing tip/sensor head has to follow the surface variations of an object very precisely in order to ensure even or equal and continuous contact with the object for an exact determination of the surface topography. Because of the relatively large inertia of the measuring head due to its mass, very smooth or even polished surfaces and/or low scanning velocities would be required for exact measurements. If, in contrast, the stylus is designed very flexible/easily deformable, scanning velocities could be increased, but the measurement head/sensor would deliver only a smoothened/approximated representation of the object surface as a measurement result.
In U.S. Pat. No. 6,633,051 a solution is proposed where a relatively stiff stylus carrier of trumpet-like shape is connected with a relatively thin, low-mass and flexible stylus, in order to ensure high eigen frequencies of the system formed by stylus and stylus carrier combined with high flexibility of the stylus, i.e. the possibility of high scanning velocities. A probe beam from a laser light source is directed within the stylus to the tip where it impinges on a retro-reflector. A deflection of the stylus tip leads to a displacement of the reflected/returned beam and is measured/recorded with a position-sensitive detector housed in the stylus carrier. Thus, the optical monitoring system comprising the excitation light source and the detector fulfils the functionality of the tactile measurement system.
As a disadvantage of the system configuration disclosed in U.S. Pat. No. 6,633,051, the optical monitoring system including laser diode, detector and driver electronics is mounted in the stylus carrier, thus enclosing several heat sources in the closed frame formed by the stylus and its carrier.
The development of heat leads to an uncontrollable deformation of the measurement system as long as thermal equilibrium is not reached.
An essential advantage of such coordinate measuring machines is their high flexibility of operation due to a fast, often even automated exchange of the measuring probes, i.e. exchange of stylus shape and length for optimum adaptation to the object to be scanned. Any exchange of the stylus leading to a change in the thermal equilibration conditions, the inclusion of any heat sources in the frame formed by the stylus and the stylus carrier has to be avoided.
Both for CMM comprising tactile sensors and CMM equipped with optical sensors, provision of an optical monitoring system is necessary if, in case of strong acceleration forces acting on optical sensors bending of the stylus, or in case of scanning tactile measurements fast deformations of the stylus occur.
Thereby, two effects have to be considered and compensated: First, a hollow stylus of significant length, such as a carbon stylus of 300 mm length and 5 mm diameter, is subjected to a static bending by about 80 μm upon horizontal orientation due to gravitational force. Additionally, this static bending deviates slightly from rotational symmetry because of eccentricity of the internal bore and material inhomogeneities of the order of some micrometers. As a second effect, in scanning measurements a dynamic bending of the stylus caused by additional acceleration forces does occur which can reach a similar amount as the static bending and which can add to or compensate the static bending. Furthermore, the stylus end can be subjected to vibrational cross-talk from actuation of the CMM.
Upon equipment with an optical monitoring system, in contrast to the configuration disclosed in U.S. Pat. No. 6,633,051, an inclusion of heat sources, i.e. of electrical power consuming devices as potential sources of further disturbing deformations, in the closed measurement system comprising stylus and stylus carrier should be avoided.